\begin{tikzpicture}[>=Stealth, scale=0.8]
    \coordinate [label=90:$A$] (A) at (0,3);
    \coordinate [label=270:$B$] (B) at (-1,0);
    \coordinate [label=270:$D$] (D) at (0,0);
    \coordinate [label=270:$C$] (C) at (1.5,0);

    \draw [thin] (A) -- (B) -- (C) -- (A);
    \draw (A) -- (D);
    \draw [right angle symbol={A}{D}{C}];
\end{tikzpicture}
